The ultimate goal of mobile broadband is ubiquitous and sustainable provision of unlimited data rates to anyone or anything at anytime. Ultra Dense Network (UDN) is a promising next step to the successful introduction of Long Term Evolution (LTE) for wide area and local area accesses. The UDN can be deployed in areas with high traffic consumptions and thus provide an evolution towards the above goal. Due to overprovision of access nodes and thus low average load in the access network, the UDN creates ubiquitous access opportunities for providing users with desired data rates even under realistic assumption on user density and traffic.
The overprovision is achieved by an extremely dense grid of access nodes. Inter-access-node distances in the order of tens of meters or below are envisioned. In in-door deployments, one or more access nodes are possible in each room. In addition to increased network capacity, densification (via reduced transmit powers) also enables access to vast spectrums in millimeter-wave bands and thus increased data rates.
As the very first step of communication, synchronization is critical to the UDN. Compared with access link synchronization between an Access Node (AN, e.g., an evolved NodeB (eNB)) and a User Equipment (UE), it is more challenging to achieve backhaul link synchronization between ANs, which is necessary for avoiding collisions between uplink and downlink (when Time Division Duplex (TDD) is applied) and achieving intelligent inter-cell interference coordination (e.g., enhanced Inter-Cell Interference Cooperation (eICIC)). In traditional cellular networks, the backhaul link synchronization is achieved via wired connections, including e.g., packet based synchronization (Network Time Protocol (NTP) or Precision Time Protocol (PTP) (IEEE1588)) or Global Navigation Satellite System (GNSS) based synchronization (Global Positioning System (GPS) or Galileo). However, these solutions are not applicable in the UDN where ANs are deployed in an in-door scenario with wireless backhaul links.
Simeone, Spagnolini, Bar-Ness and Strogatz, Distributed Synchronization in Wireless Networks, IEEE Sig. Proc Magazine, 2008, discloses a solution for distributed synchronization in a wireless network. FIG. 1 shows a scenario where this solution is applied. As shown, each node broadcasts a synchronization signal to all of its neighboring nodes and each node updates its local timing value based on the synchronization signals received from all of its neighboring nodes. This solution requires a number of iterations before the timing values of the nodes converge.
However, the distributed synchronization solution is adversely affected by propagation delay of the synchronization signals, which leads to timing and phase errors.
Conventionally, the effect of the propagation delay can be mitigated by means of timing advance update. For a link between a pair of nodes, the propagation delay over the link can be mitigated by exchanging timing information between the nodes, estimating the propagation delay based on the timing information and removing the effect of estimated propagation delay from the timing values of the nodes. However, the increase in signaling overhead required for exchanging the timing information between each pair of nodes may be significant, especially when there are a large number of nodes e.g., in an UDN.
There is thus a need for an improved solution for distributed synchronization.